Radar apparatus transmit a radio-frequency radar transmission signal to the space from a measuring site, receive a reflection wave signal reflected from a target, and measure at least one of a distance between the measuring site and the target and a direction of the target. In recent years, demand for radar apparatus which can estimate, at a high resolution, a distance to or a signal incoming direction (in a wide range) from a target that may be an automobile or a pedestrian using a short-wavelength radar transmission signal such as a signal of microwaves or millimeter waves has increased.
Among conventional radar apparatus are ones which estimate a signal incoming direction at a higher resolution than the resolution of the beam directivity of each reception antenna by receiving a reflection wave signal reflected from a target with an array antenna and measuring phase differences between reception signals received by the respective reception antennas.
Such radar apparatus can estimate a signal incoming direction at a high resolution by performing signal processing using phase differences between reception signals received by the respective reception antennas, and can estimate a signal incoming angle following a target movement even though it is of a high speed.
In conventional radar apparatus, a Fourier method and a Capon method, for example, are known as signal incoming direction estimation methods using phase differences between reception signals received by an array antenna. In the Fourier method, a radar apparatus calculates a correlation matrix of reception signals received by respective reception antennas and estimates, as a signal incoming direction, an azimuth angle that gives a peak value of an evaluation function which uses the correlation matrix. In the Capon method, a radar apparatus calculates an inverse matrix of a correlation matrix of reception signals received by respective reception antennas and estimates, as a signal incoming direction, an azimuth angle that gives a peak value of an evaluation function which uses the inverse matrix of the correlation matrix.
The Fourier method can decrease the amount of calculation because it is not necessary to calculate an inverse matrix of a correlation matrix. However, large sidelobes in the cross-range direction to make discrimination from a case that plural targets having different reception levels exist difficult, as a result of which the target detection performance in the azimuth angle direction is lowered. The sidelobes in the cross-range direction mean sidelobes that appear at respective distances from a measuring position in reception signal evaluation function profiles (hereinafter referred to as “azimuth profiles”) each corresponding to an incoming direction (azimuth angle) of a reflection wave signal reflected from a target. On the other hand, in the Capon method, whereas sidelobes in the cross-range direction mean sidelobes can be made small, it is necessary to calculate an inverse matrix of a correlation matrix, resulting in increase of the calculation amount of the radar apparatus.
For example, Patent documents 1 and 2 are known as disclosing conventional techniques for reducing sidelobes. In the discrete aperture antenna apparatus disclosed in Patent document 1, a covariance matrix of reception signals received by plural reception antennas which are arranged irregularly with every element interval being longer than or equal to a half wavelength and Capon weights for minimizing output power are calculated using the covariance matrix and a steering vector. Furthermore, in the discrete aperture antenna apparatus, the reception signals are multiplied by the respective Capon weights and resulting signals are added together, whereby grating lobes (sidelobes) are suppressed that are caused by the fact that the element intervals between the reception antennas are longer than or equal to the half wavelength.
In the radar apparatus disclosed in Patent document 2, a first angular spectrum and a second angular spectrum are calculated by multiplying reception signals received by plural reception antennas by a first coefficient group and a second coefficient group, respectively. The first coefficient group consists of coefficients for reducing left-side sidelobes (see FIG. 18(A)) and the second coefficient group consists of coefficients for reducing right-side sidelobes (see FIG. 18(B)).
The radar apparatus judges whether an object exists in an angular range where the level is higher than a threshold value that is set for each of the first and second angular spectra. FIG. 18 shows example azimuth profiles in a conventional radar apparatus. FIG. 18(A) shows an azimuth profile in which sidelobes are reduced on the left side of the azimuth angle 0°. FIG. 18(B) shows an azimuth profile in which sidelobes are reduced on the right side of the azimuth angle 0°.